| 摘要 |
The walls which form a packing container are made from an extruded polystyrene foam plastic material which is stiffened along variously positioned rectilinear zones in the surface by effecting an increase in thickness of the plastic along those zones. The foamed plastic material is initially of uniform thickness during formation of the container but those zones of the walls which are to be thickened have a higher cell density factor than the remaining portions of the walls and hence a higher latent expansion capacity which, after the container has been formed, is released by heating to such an extent that the cell walls soften and are expanded by the increased gas pressure within them. The foamed plastic packing material is formed by extrusion through a die and is of uniform thickness. The desired zones of higher density are established in the material by subjecting those zones to a cooling operation.
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